During his ministry, Jesus used leaven to represent both good and bad in his lessons. Comparing it to the kingdom of heaven (Matthew 13:33, Luke 13:20-21) and to the influence of the Pharisees (Luke 12:1-3).
Where you spend your time, what friends you choose, and the habits you practice all contribute to who you are. The things we allow to influence us will spread and overtake other qualities, just like leaven causing the dough to rise.
Fill your life with good things!